Two main techniques are currently used to detect charged particles, or ions. In the present document, the expression “detecting charged particles” means “determining the presence of at least one charged particle, or the absence of any charged particle”.
The first one is based on FTICR (“Fourier Transform Ion Cyclotron Resonance”) mass spectrometry. Ions are trapped by a homogeneous static magnetic field of a Penning trap, and excited at their resonant cyclotron frequencies by a spatially inhomogeneous static electric field of said Penning trap. Ions of same mass and same mass-to-charge ratio M/Z are then rotating at their cyclotron frequency, as a “packet” of ions, and generate a measurable electric signal. The Fourier transform of this electric signal gives access to a mass spectrum. FTICR mass spectrometry is a high resolution technique but is limited to the detection of particles having a mass less than or equal to 106 Da, due to the required amplitude of the magnetic field needed to trap the particles.
The second technique is based on TOF (“Time-Of-Flight”) mass spectrometry, in which an ion's mass-to-charge ratio M/Z is determined via a time measurement. Ions are accelerated by an electric field of known strength. After this acceleration, all ions of same charge have the same kinetic energy, The velocity of the ions depends on their mass-to-charge ratio M/Z. The time that it takes for the ions to reach a detector at a known distance is measured. Mass-to-charge ratios M/Z of ions are obtained from this time and the known experimental parameters. The ions are however destructed when detected: ion detection indeed involves bombarding the ion on a semi-conductor surface, causing the removing of electrons of said surface. The signal associated to the removed electrons is amplified for example with an electron multiplier, and an observable signal is thus generated for each ion bombarding the surface. Removing electron(s) from the semi-conductor surface also implies a minimal impulsion for the ions; said minimal impulsion can be given to ions having a mass less than 106 Da.
A high resolution mass spectrometry method and system for analysis of whole proteins and other large molecules is described in patent US 2008/0230692 A1. This document aims in particular at providing an injection method of ions inside an ion trap, in order to extend the working mass range of an ion trap mass spectrometer and thus to allow real-time analysis of large molecules having a mass over 20 kDa. The proposed injection method is based on a MALDI (“Matrix-Assisted Laser Desorption Ionization”) technique. Document US 2008/0230692 A1 then uses TOF mass spectrometry: ions are fragmented by collision with a high-temperature surface, fragments are then ionized by electron impact ionization, and ionized fragments are finally detected with a classic electron multiplier. The proposed method is thus destructive for the detected ions. Electron impact ionization is further highly dependent of the nature of the fragments, inducing that the efficiency of the detection is not constant according to the ions to be detected. The proposed method is therefore inappropriate for quantitative and comparative measurements of two different types of ions.
In this context, there is a need for a method allowing ion detection without mass limitation. There is also a need for a method allowing non-destructive ion detection without mass limitation. There is also a need for a method allowing non-destructive single ion detection without mass limitation. There is also a need for a method allowing non-destructive single ion detection without mass limitation and independently of the nature of the ion.